Downstream of a transverse cutting or slicing machine, i.e. a cross-cutter dealing with cardboard and the like, the transversely cut sheets are generally stacked upon pallets which can be mounted on a raisable and lowerable sheet-collection platform.
In order to ensure sufficient time to allow a completed stack to be removed from the platform, German Patent 38 23 806 describes a stacking unit which utilizes a bar grate as an auxiliary stacking platform. The oncoming sheets are collected on this bar grate during the period in which the completed stack on its pallet is removed, a new pallet is placed upon the platform and this pallet is raised below the bar grate for transfer of the sheets which have collected thereon and incipiently forming a new stack onto that platform.
In the stack of cardboard sheets which can have a weight of more than 150 g/m.sup.2 the bar grate can be vertically fixed at a given distance below the sheet feed plane since such sheets, unlike more sensitive paper sheets, can have a relatively long freefall without creating problems in stacking while the partial stack which newly forms upon the bar grate rises in height to the level of the feed plane, the stack removal and pallet change can be carried out.
The drawback of this process and the conventional apparatus used for that purpose is that to introduce the bar grate in the sequence of sheets which generally are fed in a shingled or overlapping pattern, a gap in the shingled stream of the sheets must be ensured. The bars of the bar grate cannot readily be moved through the stack as sheets deposit thereon. The weight of the sheets collecting on the bar grate during the insertion movement of the latter continuously increases during that movement so that at least at the last part of the insertion stroke and immediately ahead of any stop board against which the sheets impact, damage to the edges of the sheets can occur by this insertion movement.
In the past efforts have been made to avoid this drawback by partially interrupting the sheet feed and for that purpose that the movement of the sheets must be impeded somewhere is itself a major drawback.
While we consider German Patent 38 23 806 to best represent the prior art, mention can also be made of DE-OS 29 42 965 which describes a stacking arrangement, especially for printing processes which may be used to stack very thin paper sheets for signatures. In this arrangement as well, a bar grate is provided. The bars of this bar grate are provided on the outlet openings for compressed air which are directed upwardly toward the leading edges of the oncoming sheets and which thus provides an air cushion supporting the sheets and assisting in the insertion of the bar grate. This arrangement, of course, is dependent upon the continuity of supply of the compressed air.
Mention should also be made of German utility model DE GM 88 04 066 which describes a vibrator for sheet stacking machines in which a vibrating plate has at its lower end rollers with axes transverse to the vibration direction and which cooperates with a spring plate extending horizontally beneath the roller 15. The spring plate acts as a running surface for the roller with which the vibrating plate is braced against the pallet. This system does not deal with the problem of insertion of a bar grate as an intermediate stacking element during replacement of the pallet on a stacking platform.
U.S. Pat. No. 2,733,064 issued Jan. 31, 1956 to George A. Martin discloses a sheet-delivering apparatus especially sheets of tin plate and other heavy metallic sheets which utilizes a jogging device to facilitate alignment of the oncoming sheets with the sheets of the stack. Here too the problem of insertion of a bar grate between a stack and shingled sheets cardboard or the like is not dealt with.
German open application DE-OS 37 21 393 describes an apparatus for stacking sheets in conjunction with cross cutting of paper or paperboard and describes a raisable and lowerable platform upon which the pallet can be placed, as well as the bar grate which can be inserted to allow removal of the completed stack. The problem of gapping the oncoming sheets to allow insertion of the bar grate is solved in a different manner in this system.